docs: write CLI docs for all 12 previously undocumented tools

Add docs/cli/ entries for:
  transport_cli, discovery_cli, config_cli, protocol_cli, query_cli,
  test_image_cli, xorg_cli, v4l2_view_cli, stream_send_cli,
  stream_recv_cli, reconciler_cli, controller_cli

Each doc covers: description, build instructions, full usage with all
options and defaults, example output, and a relationship note pointing
to related tools. controller_cli includes the display control IDs table
and notes its temporary status.

README.md: convert all CLI tool entries to links.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/cli/discovery_cli.md

@@ -0,0 +1,53 @@
+# discovery_cli
+
+A development tool for testing the UDP multicast discovery layer. Announces a node on the local network and prints peers as they appear and disappear.
+
+---
+
+## Build
+
+From the repository root:
+
+```sh
+make cli
+```
+
+The binary is placed in `build/cli/`.
+
+---
+
+## Usage
+
+```sh
+discovery_cli <name> <tcp_port> [flags]
+```
+
+| Argument | Description |
+|---|---|
+| `name` | Node name in `namespace:instance` form, e.g. `v4l2:microscope` |
+| `tcp_port` | The TCP port this node listens on for transport connections |
+| `flags` | Comma-separated role list: `source`, `relay`, `sink`, `controller` (default: `source`) |
+
+Example — announce a source node and watch for peers:
+
+```sh
+./discovery_cli camera:0 8000 source
+```
+
+Example output as peers are found and lost:
+
+```
+found: unnamed:0  192.168.1.42:8001  site=0  flags=source
+found: display:0  192.168.1.55:8002  site=0  flags=sink
+lost:  unnamed:0  192.168.1.42:8001
+```
+
+Run two instances on the same machine (different ports) to verify mutual discovery.
+
+---
+
+## Relationship to the Video Routing System
+
+`discovery_cli` exercises the `discovery` module, which the video node uses to find peers on the LAN without configuration. The node starts a discovery listener on startup; controllers use discovered peer tables to locate nodes before connecting.
+
+See also: [`query_cli.md`](query_cli.md) for a combined discovery + protocol query.